Sensors of the resistance wire or filament type have been in use for some time for measuring variable quantities such as temperature, stress, strain, torque, acceleration, pressure, etc. The sensors utilized may be of various configurations such as single elements (e.g., thermocouples) or multiple element, bridged configurations (e.g., strain gages) to provide a response proportional or related to the quantity to be measured. For example, a typical strain gage contains one or more metallic wires or filaments electrically connected in a conventional balanced bridge circuit. The force to be measured is mechanically coupled to the gage by various means, discussed below, so that the force exerted on the gage through the coupling means causes the dimensions of the wires to be altered, which results in a proportional change in the resistance of the wire. This in turn causes unbalancing of the bridge circuit by an amount at least roughly proportional to the force applied to the gage.
Many such sensors are used to monitor the temperatures and/or forces exerted upon mechanical components operating in difficult environments. As such, the sensors or gages are also exposed to the severe environmental conditions along with the high forces to be measured. While the advance of technology has improved the sensors themselves, as regards to their accuracy, reliability, etc., the attachment and coupling of these sensors to the components remains a weak link in the measurement process. It should be apparent that the sensors must not only be firmly attached so as to resist injury by the operating conditions but also firmly mechanically coupled to the component so as to efficiently transfer the temperature or forces acting thereon to the gage for accurate measurement.
The prior art has envisioned and implemented several different methods for mounting sensors to mechanical components operating in moderate to severe environments but each method has certain limitations.
For example, it is known to fabricate a sensor on a thin metallic support which is then mechanically locked to the component by pins, rivets, or even welding. See U.S. Pat. No. 3,245,018. Such methods are generally not useful for attachment to non-metallic ceramic materials.
It is also generally known to bond the sensors directly to the component by means of simple glues, cements, or similar adhesives. See, for example, U.S. Pat. No. 3,745,502. Epoxy glues are typically used since they have relatively good bonding characteristics and do permit fairly good force or strain transmission to the sensor. However, depending on the environment in which the sensor is operating, epoxy bonding is limited to testing or operating at temperatures below about 500.degree. or 600.degree. F.
Other bonding techniques, such as using glass or refractory cements, have been tried with limited success up to about 1000.degree. F. See, for example, U.S. Pat. Nos. 3,805,377 and 3,913,391. However, all such techniques have serious disadvantages when attempts are made to extend their use into more exotic fields.
One commercially important field in which the present designs or applications of sensors are becoming inadequate to perform desired measurements is in the development and testing of high temperature non-metallic materials for gas turbine engines. Although the technology base of these materials (which include monolithic ceramics, ceramic or glass matrix composites, and carbon-carbon composites) has grown significantly in the last decade, further testing of full-scale components, sub-elements, and specimens with strain gages and thermocouples is required to better characterize the thermomechanical response of these materials under typical operating conditions.
Advanced gas turbine engines contemplate complex components operating at temperatures much higher than any other type of machinery, often to several thousand degrees. Therefore, in order to reliably model, test, and integrate new materials into advanced turbines, improved methods of instrumentation attachment are required.
It is therefore an object of the present invention to provide a new and improved method and apparatus for attaching instrumentation to non-metallic components for measuring or testing such under adverse conditions.
Another object of the invention is to provide a relatively simple and easy method of firmly attaching instrumentation to ceramic components of any desired size or shape.
A further object of the invention is to provide improved methods and procedures for the manufacture, installation and operation of instrumentation at very high temperatures.